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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 2nd Oct 2010, 10:30 am   #1
'LIVEWIRE?'
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Default Bell doen't work with 12v Bell Transformer-why not?

First-is this the right section for this query-if not, please move to a more appropriate one.
Second-the bell in question is a vintage 12v Bell housed in a wooden case.
The DC resistance of the coil is about 11 ohms, and when tested on my stabilised variable DC PSU it draws about 350-500mA at voltages ranging from 11-15v, yet a 1amp Bell Transformer will not power it. I realize that (a) the current drawn is pulsed as the bell rings, and (b) that the transformer is purely that -i.e. it's o/p is 12vAC @ 12vA. In theory that says that the bell SHOULD work, since it's average current consumption is only half the rating of the transformer(although by ohm's law it should consume about 1.1Amps!)
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Old 2nd Oct 2010, 10:38 am   #2
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Could be the bell coils have too much inductance and don't draw enough current on AC to attract the armature.
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Old 2nd Oct 2010, 10:42 am   #3
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

The bell in my servant bell board won't work on AC either.

How about attaching a full wave rectifier and maybe a smoothing cap to the transformer and seeing if this makes any difference.
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Old 2nd Oct 2010, 10:47 am   #4
kalee20
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

In fact a smoothing capacitor may be unnecessary. With unsmoothed DC from a bridge rectifier, the bell coil inductance will allow it to draw current corresponding with the average (not RMS) transformer voltage minus diode drop, divided by coil resistance.
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Old 2nd Oct 2010, 11:21 am   #5
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Hadn't thought about the effects of inductance, so maybe adding a rectifier will solve the problem. Thanks, both, for the suggestions.
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Old 2nd Oct 2010, 2:36 pm   #6
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Some countries on the Continent use a special 16.7Hz AC supply for their railways, for this very reason: the inductance of the (compound-wound) traction motors prevents enough current from flowing at 50Hz.
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Old 2nd Oct 2010, 3:49 pm   #7
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Hi

I used to have a big wood cased bell with a 6" polished brass dome as a front door bell (in my last house, left it behind when I sold), it would only work well on DC, with a smoothing cap.

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Old 2nd Oct 2010, 4:45 pm   #8
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Quote:
Originally Posted by ajs_derby View Post
'Some countries on the Continent use a special 16.7Hz AC supply for their railways...'
Wonder if this is the origin of 16.7Hz for magneto bell telephone ringing?

A.c. bells are designed with fatter wire on the bobbins (and lower resistance), but it is possible to get some d.c. bells to work on a.c. by connecting to the bobbins directly to the supply and adjusting the trembler contact (now nothing more than a back-stop) to move the armature nearer the pole-pieces. Not all, though...
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Old 2nd Oct 2010, 5:24 pm   #9
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

Quote:
Originally Posted by Station X View Post
The bell in my servant bell board won't work on AC either.
The one on mine does, but I run it at a higher voltage than I would at d.c. A well-designed bell (manufactured by Gents, typically) will be rated at 4.5 - 6V d.c. and 8V a.c.

I find a.c. on a trembler bell gives a coarser ring.
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Old 2nd Oct 2010, 6:26 pm   #10
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Default Re: Bell doen't work with 12v Bell Transformer-why not?

I think my board was made in the 1920s and probably designed to work on Leclanche or flag cells. No matter how much I adjust it, it won't work on less than 9V DC and is much happier on 12V. The indicators are wired in series with the bell. 12V is a lot of flag cells, but I intend to make some dummy ones and put them in a suitable rack.
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